This invention relates to series-shunt diode switches for microwave circuits and methods of fabricating such series shunt diode switches.
In a series-shunt diode switch, the series diode (connected between an input port and a switched output port) and shunt diode (connected across the output port and a ground plane) are controlled by a common control signal applied to the junction between the two diodes (e.g., the cathode of the series diode and the anode of the shunt diode). The control signal switches the diodes to opposite conducting states. With the series diode switched "ON" (conducting) and the shunt diode "OFF" (nonconducting), a signal at the input port is coupled to the output port through the low resistance (typically about 4 ohms) of the series diode. Conversely, with the series diode switched "OFF" and the shunt diode "ON", the input signal is decoupled from the output port with an isolation of 30 dB or more.
One known method of fabricating the switch is to use discrete, separately packaged PIN diodes, such as a beam lead-type series diode and a chip-type shunt diode. Another conventional technique is to fabricate each discrete diode and, before packaging, connect the beam lead of, for example, the cathode of the series diode directly to the anode of the chip shunt diode.
The minimum spacing between the pair of packaged diodes is on the order of 3/4 mm; the spacing is reduced by about half for the discrete diodes in one package. Thus, while either of these fabrication method may be satisfactory in some applications, in microwave circuits such spacing between the diodes may adversely affect such circuit parameters as isolation, insertion loss, and VSWR.